1. Field of the Invention
The present invention relates to an organometallic complex. In particular, the present invention relates to an organometallic complex that is capable of converting a triplet excited state into luminescence.
2. Description of the Related Art
Organic compounds are brought into an excited state by absorption of light. Through this excited state, various reactions (photochemical reactions) are caused in some cases, or luminescence is generated in some cases. Therefore, the organic compounds have a wide range of applications.
As one example of the photochemical reactions, a reaction of singlet oxygen with an unsaturated organic molecule (oxygen addition) is known (see Non-patent Document 1, for example). Since the ground state of oxygen molecules is a triplet state, oxygen molecules do not form a singlet state (singlet oxygen) when they are excited directly by light. In contrast, when oxygen molecules interact with triplet excited molecules other than oxygen, the oxygen molecules form singlet oxygen, thereby leading to an oxygen addition reaction. Here, the compound that forms the triplet excited molecules by light and enables formation of singlet oxygen is called a photo sensitizer.
Thus, formation of singlet oxygen requires a photosensitizer that can form triplet excited molecules by light. However, since the ground state of an ordinary organic compound is a singlet state, formation of a triplet excited state by light is forbidden transition; thus, triplet excited molecules are difficult to generate. Therefore, as the photosensitizer, it is necessary to use a compound that can easily cause intersystem crossing from the singlet excited state to the triplet excited state (or a compound that allows the forbidden transition through excitation directly by light into the triplet excited state). In other words, such a compound can be used as the photosensitizer and is useful.
Further, such a compound often exhibits phosphorescence. Phosphorescence refers to luminescence generated by transition between different energies in multiplicity. In an ordinary organic compound, phosphorescence refers to luminescence generated in returning from the triplet excited state to the singlet ground state (in contrast, fluorescence refers to luminescence in returning from the singlet excited state to the singlet ground state). Application fields of a compound capable of exhibiting phosphorescence, that is, a compound capable of converting the triplet excited state into luminescence (hereinafter, referred to as a phosphorescent compound), include a light-emitting element including an organic compound as a light-emitting substance.
An example of the structure of such a light-emitting element is a simple one in which a light-emitting layer containing an organic compound that is a light-emitting substance is merely provided between electrodes. Light-emitting elements having such a structure can achieve thinness, lightweight, high-speed response to signals, low-voltage DC drive, and the like. Therefore, attention has been directed to the light-emitting elements as next-generation flat panel display elements. Further, displays using such light-emitting elements are superior in contrast, image quality, and wide viewing angle.
The light-emitting element including an organic compound as a light-emitting substance has a light emission mechanism of a carrier injection type: a voltage is applied between electrodes where a light-emitting layer is interposed, electrons and holes injected from the electrodes recombine to make the light-emitting substance excited, and then light is emitted in returning from the excited state to the ground state. As in the case of excitation described above, types of excited state include a singlet excited state (S*) and a triplet excited state (T*). The statistical generation ratio thereof in light-emitting elements is considered to be S*:T*=1:3.
At room temperature, a compound capable of converting a singlet excited state to luminescence (hereinafter, referred to as a fluorescent compound) exhibits only luminescence from the singlet excited state (fluorescence), not luminescence from the triplet excited state (phosphorescence). Therefore, the internal quantum efficiency (the ratio of generated photons to injected carriers) of a light-emitting element including a fluorescent compound is assumed to have a theoretical limit of 25%, on the basis of S*:T*=1:3.
On the other hand, in the case of a light-emitting element including a phosphorescent compound described above, the internal quantum efficiency thereof can be improved to 75% to 100% in theory; i.e., the emission efficiency thereof can be 3 to 4 times as much as that of a light-emitting element including a fluorescent compound. Therefore, light-emitting elements including a phosphorescent compound have been actively developed in recent years in order to achieve highly-efficient light-emitting elements (e.g., see Non-Patent Document 2). Organometallic complexes that contain iridium or the like as a central metal have particularly attracted attention as phosphorescent compounds because of their high phosphorescence quantum yield.